Single Molecule Studies of Interaction Between Alzheimer's Amyloid-β Peptides of different Lengths

Abstract

The oligomers formed by amyloid-β (Aβ) peptides are thought to play a causative role in Alzheimer's disease, possibly due to membrane permeabilization by small Aβ oligomers. Two primary variants of Aβ, of lengths 40 (Aβ40) and 42 (Aβ42) amino acids are produced at defined ratios in a normal individual. Changes in the ratio of Aβ40/Aβ42 have been shown to correlate strongly with the disease. Further, a mixture of the two forms of the peptide has been shown to exhibit different fibrillization kinetics in vitro and to elicit different extents of cellular toxicity. Thus, mixed Aβ40/Aβ42 oligomers can have different characteristics compared to oligomers formed by either one of the peptides. However, most research in the field focuses on one of the peptides at a time. Further, the Aβ oligomers are heterogenous, metastable and physiologically occur at low nanomolar concentrations, which makes it difficult for the use of conventional techniques to identify the toxic oligomers. In our work, we use single molecule methods to overcome these obstacles and study the formation and the evolution kinetics of mixed oligomers of Aβ40 and Aβ42 at different ratios in solution. We employ single molecule FRET and photobleaching of two different fluorophores attached to the N-terminal of Aβ40 and Aβ42 to determine the oligomer size and composition. Extending these studies to oligomer formation and binding kinetics of the two peptides on model membranes constructed from brain lipid extracts will provide a critical new understanding of how the stoichiometry of interaction of the two peptides both in solution and on the membrane surface affects the composition and permeabilizing potency of the resulting mixed oligomers.